1. Field of the Invention
This invention relates to the recovery of gold from sedimentary, carbonaceous gold-containing ores which are refractory to standard cyanidation techniques. In the context of this disclosure "carbonaceous gold-containing ores", or, simply, "carbonaceous ores", denote refractory gold-containing ores which also contain organic carbonaceous matter which exhibits the property of inhibiting or substantially reducing the extraction of gold from such ores by conventional cyanidation technology. The invention relates, more particularly, to an improved process for the treatment of these ores which makes use of simultaneous cyanidation and carbon adsorption to obtain consistently high gold recoveries.
2. The Prior Art
Sedimentary carbonaceous gold-bearing ores are found in Nevada, Utah, California and other states in the United States, as well as other countries throughout the world. These ores are not amenable to standard cyanidation techniques because the carbonaceous impurities with which they are associated tend to tie up the cyanide gold complexes in chemical compounds from which the gold cannot be separated by standard methods. Also, gold may be associated with organometallic complexes in the ore which are not attacked by the cyanide complexing agents used in conventional cyanidation techniques. Conventional cyanidation techniques can normally recover only up to about 50 percent of the gold which is present in these ores.
Prior art methods for dealing with the problem of carbonaceous impurities in gold-containing ores have been confined, for the most part, to various treatments of the ore prior to cyanidation in attempts to make it more amenable to the cyanide leaching action. Thus, for example, U.S. Pat. No. 1,461,807 discloses the use of certain mineral oils for "blinding" the action of the carbonaceous impurities on the cyanide complex formation; U.S. Pat. No. 2,234,140 teaches that certain wetting agents can make the ore more amendable to cyanidation; U.S. Pat. No. 3,639,925 discloses the use of sodium hypochlorite and calcium hypochlorite as agents for oxidizing the carbonaceous material so as to prevent it from absorbing the gold cyanide; and U.S. Pat. No. 3,846,124 calls for a chlorine pretreatment of the ore, in the absence of any alkaline material, in order to decompose the organic carbonaceous components and remove them prior to cyanidation. In addition, U.S. Pat. No. 3,574,600 teaches that certain acids can be used in conjunction with an ozone treatment prior to cyanidation to oxidize the carbonaceous impurities; and U.S. Pat. No. 4,038,362 discloses a preoxidation technique, which is carried out in the absence of extraneous alkaline material, for reducing the amount of chlorine needed to pretreat the ore, as in the '124 patent. Alternatively, calcining of the ore, prior to cyanidation, has also been suggested as a way of oxidizing all of the organic constituents and thus prevent the carbon from interfering with the cyanide leaching action. Calcining operations, of course, tend to generate mercury, arsenic and sulfur-containing gases, the release of which to the atmosphere is extremely undesirable from an environmental standpoint.
Methods which involve treatment during cyanidation include those of U.S. Pat. Nos. 2,147,009 and 2,315,187, which cover the use of finely divided charcoal during cyanidation to simultaneously leach the gold values from the ore and adsorb them on the charcoal, so as to maintain the solution continuously depleted of gold and thereby improve cyanidation efficiency.
These prior art methods have proven satisfactory in some cases, but have not been able to provide a solution to the problems caused by the organic carbon in some of the more refractory ores such as the ores found in the Jerritt Canyon and Marlboro Canyon areas of Elko County, Nevada, and other equally refractory carbonaceous ores. Thus the methods of U.S. Pat. Nos. 1,461,807 and 2,234,140 work well on ores when inorganic carbon predominates, but do nothing or very little to alleviate the problems caused by adsorption of the gold cyanide complex as a result of the presence of organic carbon in ores such as those contemplated by the process of this invention. Likewise, the method of U.S. Pat. No. 3,639,925 provides only a partial solution to the problem of handling very refractory ores because, even though the hypochlorite treatment makes them more amenable to cyanidation, recoveries are not satisfactory unless high levels of hypochlorite are used. Chlorination methods such as those of U.S. Pat. No. 3,846,124 do not yield very good recoveries in spite of the fact that they call for very high consumption of chlorine or hypochlorite and are, consequently, very expensive, usually necessitating additional steps such as the preoxidation step of U.S. Pat. No. 4,038,362, which entails heating the slurry to elevated temperatures while bubbling air for long periods of time.
The method of U.S. Pat. No. 3,574,600 consumes expensive ozone and acid and has essentially the same effect as the method of U.S. Pat. No. 3,639,925 with its attendant shortcomings. In addition, it must be carried out at very low pH values, which makes it unsuitable for treating calcium carbonate-containing ores such as the sedimentary ores treated by the process of this invention.
Finally, the methods which deal with treatment during cyanidation, that is, those of U.S. Pat. Nos. 2,147,009 and 2,315,187, treat oxide type ores, not carbonaceous ores, and use finely divided charcoal, not granular activated carbon. These methods do not work on the carbonaceous ores of this invention, partly because the carbonaceous material present in the ores of this invention inhibits the dissolution of gold complexes and prevents finely divided charcoal from removing the gold from the cyanide leach solution. In addition, finely divided charcoal cannot be separated by means of screening.
Obviously, a process is needed that can recover the gold values from ores containing substantial quantities of organic carbonaceous matter without consumption of large amounts of expensive reagents such as chlorine.
It is an important object of this invention to provide an economical process for treating highly refractory, carbonaceous, gold-containing ores which does not suffer from the disadvantages of prior art processes and which, at the same time, results in high yields of gold extraction from said ores.